1. Field of the Invention
The present invention relates to a new technique for digitally encoding and decoding in particular but not exclusively speech signals in view of transmitting and synthesizing these speech signals.
2. Brief Description of the Prior Art
Efficient digital speech encoding techniques with good subjective quality/bit rate tradeoffs are increasingly in demand for numerous applications such as voice transmission over satellites, land mobile, digital radio or packed network, for voice storage, voice response and secure telephony.
One of the best prior art methods capable of achieving a good quality/bit rate tradeoff is the so called Code Excited Linear Prediction (CELP) technique. In accordance with this method, the speech signal is sampled and converted into successive blocks of a predetermined number of samples. Each block of samples is synthesized by filtering an appropriate innovation sequence from a codebook, scaled by a gain factor, through two filters having transfer functions varying in time. The first filter is a Long Term Predictor filter (LTP) modeling the pseudoperiodicity of speech, in particular due to pitch, while the second one is a Short Term Predictor filter (STP) modeling the spectral characteristics of the speech signal. The encoding procedure used to determine the parameters necessary to perform this synthesis is an analysis by synthesis technique. At the encoder end, the synthetic output is computer for all candidate innovation sequences from the codebook. The retained codeword is the one corresponding to the synthetic output which is closer to the original speech signal according to a perceptually weighted distortion measure.
The first proposed structured codebooks are called stochastic codebooks. They consist of an actual set of stored sequences of N random samples. More efficient stochastic codebooks propose derivation of a codeword by removing one or more elements from the beginning of the previous codeword and adding one or more new elements at the end thereof. More recently, stochastic codebooks based on linear combinations of a small set of stored basis vectors have greatly reduced the search complexity. Finally, some algebraic structures have also been proposed as excitation codebooks with efficient search procedures. However, the latter are designed for speed and they lack flexibility in constructing codebooks with good subjective quality characteristics.